PHARMACY DEDUCTIBLES CAN COMPLICATE THE RELATIONSHIP BETWEEN MEASURES OF PATIENT COST SHARING AND MEDICATION ADHERENCE

DISCLOSURES: This letter pertains to our recent publication in JMCP, which describes a study that was jointly funded by the Pharmacy Quality Alliance and the National Pharmaceutical Council.

Following the publication of our article "Predictors of adherence to oral anticancer medications: An analysis of 2010-2018 US nationwide claims" in the August 2022 issue of JMCP, 1 we received a personal communication from a peer researcher about classification bias resulting from our method for calculating mean monthly out-ofpocket (OOP) cost and its relationship with medication adherence.
We determined the mean patient OOP cost for oral anticancer medications (OAMs) by summing copays and deductible payments for each prescription claim during a 6-month period. Average OOP costs per 30-day supply of an OAM were calculated by dividing the total OOP costs by the total days of medication supply during the 6-month follow-up period and then multiplying by 30. This approach was informed by other research published in JMCP. 2 For the multivariable modeling, we dichotomized OOP spending at the highest quartile of OOP spending.
The bias of concern relates to deductible payments being higher for initial dispensings and lower after the deductible was met. This dynamic is problematic when assessing the relationship between mean monthly OOP costs and medication adherence because some patients with fewer dispensings will have higher OOP costs due to the role of deductibles.
To examine this issue, we performed a sensitivity analysis for a subgroup of patients with blood cancers. This cohort had the highest percentage of patients with a pharmacy deductible among all cancer types assessed in our study: 24.8% (1620/6523). After excluding cases with a deductible, the adjusted odds ratio for the relationship between mean monthly OOP costs and nonadherence was 3.15 (95% CI = 2.62-3.80), compared with an adjusted odds ratio of 2.89 (95% CI = 2.48-3.37) for the original analysis. Both the results of the sensitivity analysis and the original analysis found that those in the highest quartile of OOP spending were roughly 3 times more likely to be nonadherent, controlling for covariates.
In our study, the bias resulting from declining deductible payments over time may have been limited by 3 factors. First, for the database that we analyzed, most patients with blood cancer (75.2%) did not have a pharmacy deductible, and 83% of the patients with a deductible were enrollees of Medicare Advantage, for which deductibles were modest. Only 4.2% were patients in a commercial plan with a higher deductible. Second, the high cost of oral anticancer medications may have overpowered the bias resulting from declining deductible amounts over time. Third, dichotomizing mean monthly OOP costs at the fourth quartile of spending may have attenuated the effect of deductible payments.
Although the sensitivity analysis reinforced our original findings, we nevertheless thought it important to update our limitations to note that the approach we used to calculate average monthly OOP costs should be applied with caution. Future research on this topic should be mindful of the role of deductible payments, which may be more influential in other studies, particularly given the increased use of pharmacy deductibles and benefit designs that eliminate cost sharing after the deductible is met. 3

DISCLOSURES
This letter pertains to our recent publication in JMCP, which describes a study that was jointly funded by the Pharmacy Quality Alliance and the National Pharmaceutical Council.